The IS-95 standard describes a communication protocol to be carried out using code division multiple access ("CDMA") processing. Wireless communications standards, such as the IS-95 communications standard, will allow transmission at up to 64 kilobits per second ("KB/s") on forward and reverse links. This data transmission uses multiple code channels. Each code channel carries a specified data rate. The proposed system is shown in FIG. 1.
The ratio between the peak value and the average value of the transmitted signal in such a system should be kept as small as possible in order to minimize the peak power output of the transmitter chain. However, all of the multiple code channels 100, 130, 132 convey information which may be correlated to some extent. The information in those code channels are eventually added together by adder 125 to form a composite signal 120. That signal would have a high peak to average power ratio in the transmitted signal.
The IS-95 standard suggests minimizing this problem by independently phase shifting with respect to the fundamental channel each of the supplemental codes channels S.sub.l (t) to S.sub.n (t). These phase shifts are fixed by the IS-95 standard as follows:
TABLE I PHASE SHIFT ANGLES FOR SUPPLEMENTAL CODE CHANNELS Supplemental Code Carrier phase offset .phi., Channel I (radian) 1 .pi./2 2 .pi./4 3 3.pi./4 4 0 5 .pi./2 6 .pi./4 7 3.pi./4
This modification attempts to minimize the peak to average ratio. However, it does so at the expense of significant hardware/signal processing requirements.
A fundamental code channel 100 is combined with a plurality of supplemental code channels. Two supplemental code channels 130, 132 are shown. The total number of supplemental code channels can actually vary between n=1 and 7. The fundamental channel is used to transmit voice while the supplemental channels transmit coded information i.e., (data). Channel 100 transmits its data "in phase" and "quadrature" modulated according to the component cosine or sin of 2.pi.f.sub.c t. Each of the code channels are formed independently using distinct I and Q code sequences which are respectively modulated according to the cos and sin.
The I and Q code sequences are baseband-filtered by respective filters 102, 104. The baseband filters are offset-quadrature phase shift keying ("O-QPSK") modulators. These modulators operate as known in the art. The thus-modulated signals are then added to form a composite signal 106; also called s.sub.n (t), where n is the channel number. All of the composite signals from the various phase-shifted channels are finally added by an adder 125, to form a final composite signal 120. The mobile unit transmits that composite signal. The circuit shown in FIG. 1 requires two of the baseband filters 102, 104 for each of the channels. This circuit requires 2.multidot.(n+1) baseband filters for the n+1 channels.
Moreover, this proposed circuit applies its phase shift as part of the modulation by the fundamental frequency, hence in the RF domain.
The inventor of the present invention realized that operations in the baseband domain can be carried out much more easily than operations at RF. For example, an integrated circuit which is optimized for arithmetic operations is often used in forming the coded signal. Most of the calculations at baseband can be done on such a device, e.g. a digital signal processor ("DSP") or other specialized processing device.
On the other hand, operations carried out at RF frequencies cannot be done this way. Such operations require specialized RF techniques including balancing lines and other known features.